In many diagnostic evaluations of ultrasound images, the quantitative evaluation of the tissue kinematic properties (velocity and deformation) improves the ability to identify dysfunctions. A field where this kind of analysis has a particular relevance is the field of echocardiographic diagnostic imaging. In this field, the assessment of the effective ventricular function requires the knowledge of numerous properties about the ventricular dynamics.
A recent technique for evaluating velocity is Doppler Tissue Imaging, or DTI. This technique allows the measurement of tissue velocity over all points in the ventricular wall. The measurement of velocity itself provides direct information about the wall motion and helps to uncover abnormalities not immediately observable from tissue visualisation in B-mode imaging. The velocity measurement contains information about rigid body displacement and contraction/distension, the latter being immediately related to myocardial activity. Post processing of the DTI velocity data allows evaluation of additional quantities, namely, strain-rate and strain, that are strictly related to the regional function. Segmental strain gives a direct evaluation of the degree of contractility of the myocardium during systole, as well as of its relaxation during ventricular filling.
Nevertheless, DTI suffers from a few drawbacks consisting in limitations of the technique. The evaluation of velocity, particularly when strain rate and strain are evaluated, requires a higher frame rate with respect to B-mode imaging because velocity is a more rapidly varying function than B-mode displacement. A Doppler signal requires additional processing with respect to a simple echo.
Doppler tissue imaging suffers a further intrinsic limitation due to the fact that only the component of velocity along a scanline can be measured. This limitation has several drawbacks. When tissue moves in a direction that is not aligned with the scanline, the Doppler velocity does not reflect the effective tissue kinematics. Only the component of strain and strain-rate along the scanline can be evaluated, giving a reduced view of the local deformation state. This limits the application of DTI to the anatomic sites that can be imagined aligned along a scanline. In echocardiography this corresponds essentially to the interventricular septum and to the lateral walls in apical view.
A strain rate analysis method in ultrasonic diagnostic imaging applying the above mentioned DTI technique is disclosed in WO 02/45587. According to this document, strain-rate analysis is performed for ultrasonic images in which the spatial gradient of velocity is calculated in the direction of tissue motion. Strain-rate is calculated for cardiac ultrasound images in the direction of motion which, for myocardial images, may be either in the plane of the myocardium or across the myocardium. Strain-rate information is calculated for a sequence of images of a heart cycle and displayed for an automatically drawn border such as the endocardial border over the full heart cycle. Using DTI techniques, the method of document WO02/45587 suffers the same drawbacks as the DTI technique. Furthermore WO02/45587 teaches how to carry out the automatic drawing of a border and the successive tracking of that border during its motion. In any case such a method is affected, as are all the other border detection methods that are based on arbitrary definitions of a border, by non complete reliability and thus see rare practical use in clinic diagnosis, since the imaged structures are often not so easy to be determined.
From the fluid dynamics perspective, a velocity field estimation method exists that is known as called Particle Image Velocimetry, or PIV. According to this method, a sequence of grey scale images are taken on an illuminated slice of a fluid seeded with non buoyant micro particles in order to measure the velocity of the micro-particles from the sequence of images. This method is an optical flow method and, as such, it is based on the assumption of conservation of brightness. According to this assumption, an object (i.e., a patch of brightness) displaces without local changes from one image frame to the consecutive frame. PIV is actually well suited for fluid motion where relevant deformations are present and it has been widely employed in measuring turbulent flow and shows good reliability in such extreme conditions when the explicit subject is not clearly identifiable. For better understanding of PIV, see Adrian R J, Particle-Image technique for experimental fluid mechanics, Annu. Rev. Fluid Mech. 1991; 23, 261; Melling A, Tracer particles and seeding for particle image velocimetry, Meas. Sci. Technol. 1997; 8, 1406; Singh A. Optic Flow Computation: A unified Perspective, Piscataway, N.J.; IEEE Comput. Soc. Press, 1992; Barrow J L, Fleet D J, Beuchermin S., Performance of optical flow techniques, International Journal of Computer Vision 1994; 12, 43-77; Hu H. Saga, T. Kobayashi, T. Taniguchi, N., Research on the vertical and turbulent structures in the lobed jet flow by using LIEF and PIV, Meas. Sci. Technolo. 2000; 1, 698 and Browne P, Ramuzat A, Saxena R, Yoganathan A P.
The present invention aims to provide a method for estimating tissue velocity vectors and deformation state, such as, for example, strain and shear, from ultrasonic diagnostic imaging data which does not need to acquire ultrasonic image data in Doppler mode and which can obviate the drawbacks of the velocity and strain evaluation carried out starting from the said Doppler mode ultrasonic data, while still furnishing a reliable and precise evaluation of the velocity vector and deformation tensor.
Furthermore the present invention aims to provide an evaluation of velocity vectors and strain data which allows the determination of velocity components which are transversal to the scanline, and all of the independent components of a two dimensional strain consisting of the longitudinal strain along two orthogonal axis, particularly along the tissue and across the thickness, and in the shear.